1. Field of the Invention
The present invention relates to an antenna switching apparatus for a dual-mode transceiver for use in a mobile telephone of a mobile telephone system.
2. Description of the Prior Art
A dual mode (digital mode and analog mode) mobile telephone system is available in the U.S.A., and a time division multiple access (TDMA) system is employed in the mobile telephone system.
The TDMA system will be described below.
Initially, a plurality of channels in which one frame each of is 40 milliseconds are set and each frame thereof is composed of 6 slots (slots 1 to 6). Each slot has a length of 162 symbols (=324 bits). In the first service, 2 slots (slots 1, 4; slots 2, 5; and slots 3, 6) disposed with an equal interval at every frame are employed as one channel of the full rate traffic channel and 3 channels are multiplexed on the existing high frequency one channel. In the future, for the half rate traffic channel, any one slot within each frame is employed as one channel of the traffic channel and 6 channels are multiplexed on the one high frequency channel.
In the mobile telephone, the transmission and reception frame timing is determined on the basis of the interval of one time slot+44 symbols and the transmission has a priority over the reception. This reference position is referred to as a standard offset reference. The transmission timing of the mobile telephone is controlled by the base station such that it may become earlier than the reference position by the time alignment and is adjusted so as not to conflict with other slots upon reception in the base station.
Accordingly, in the slots 1 to 6 of one frame, for example, a mobile telephone A carries out between it and the base station transmission (slot 1), reception (slot 2), idle (slot 3), transmission (slot 4), reception (slot 5), idle (slot 6), and a mobile telephone B carries out similar transmission, reception and idle of slots 1 to 6 with a delay of time of one slot from the mobile telephone A. Further, a mobile telephone C carries out transmission, reception and idle of similar slots 1 to 6 with a delay of time of one slot from the mobile telephone B. These processes are subsequently sequentially repeated.
A slot format of the mobile telephone relative to the base station is composed of guard time, lump time, data, sync. slow, associated control channel, coded digital verification color code and data, in that order. On the other hand, a slot format of the base station relative to the mobile telephone is composed of sync. slow, associated control channel, data, coded digital verification color code, data and auxiliary area. Incidentally, the data is employed as the user information area.
From the foregoing description, it is to be understood that the telephone system of the digital mode is based on the transmission and reception switching system, while the analog mode telephone system is based on the simultaneous transmission and reception system.
As the carrier frequency, the frequency band of 824 to 849 MHz is allocated to the transmission (for reception at the base station) in the mobile telephone and the frequency band of 869 to 894 MHz is allocated to the reception (transmission from the base station) at the mobile telephone, respectively. Each of these bands is divided at the interval of 30 kHz and one high frequency channel is allocated to each of the divided bands.
An example of a conventional transmitter and receiver circuit for a mobile telephone in the dual-mode mobile telephone system available in the U.S.A. will be described with reference to FIG. 1.
Referring to FIG. 1, it will be seen that a transmitting circuit 1 and a receiving circuit 2 are connected to a transmitting and receiving antenna 3 via a duplexer 20. The duplexer 20 might be a dielectric filter which is composed of a reception side bandpass filter of high Q which passes therethrough a received high frequency signal, the carrier frequency of which is, for example, 880 MHz and a transmission side bandpass filter of high Q which passes therethrough a transmitted high frequency signal, the carrier frequency of which is, for example, 835 MHz. An insertion loss of the duplexer 20 is about 2.0 dB on the transmitting side and about 2.7 dB on the receiving side.
A circuit configuration of the transmitting circuit 1 will be described. As shown in FIG. 1, a modulated audio signal applied to an input terminal 28 is supplied to and frequency-converted by a mixer circuit 26 in a frequency converter 25. The frequency converter 25 is comprised of the mixer circuit 26 and a local oscillator 27. Then, the thus frequency-converted modulated audio signal from the mixer circuit 26 is supplied through a bandpass filter 20 to a high frequency power amplifier 12 and is thereby amplified. The transmission high frequency signal from the power amplifier 12 is supplied through a circulator 11 and the duplexer 20 to the transmitting and receiving antenna 3, from which the transmission high frequency signal is transmitted.
A circuit configuration of the receiving circuit 2 will be described next. As shown in FIG. 1, a received high frequency signal from the duplexer 20 is amplified by a high frequency amplifier 9 and then supplied through a bandpass filter 8 to a mixer circuit 22 of a frequency converter 21, in which it is frequency-converted. The frequency converter 21 is comprised of the mixer circuit 22 and the local oscillator 27 which is commonly used by the transmitting circuit 1 and the receiving circuit 2. The frequency-converted modulated audio signal from the mixer circuit 22 is fed to an output terminal 24.
In the example of the conventional transmitting and receiving circuit, since the transmitted high frequency signal and the received high frequency signal are passed through the duplexer having large insertion loss in the digital mode, an amplification factor of the high frequency amplifier for amplifying the transmission high frequency signal must be increased and an amplification factor of the high frequency amplifier for amplifying the received high frequency signal must be increased, which unavoidably causes power necessary for transmission and reception to be increased by the amount of the insertion loss of the duplexer.